This invention relates to a pole and coil assembly for a dynamoelectric machine and to a method for making such an assembly; more particularly, it relates to a salient pole assembly having an improved pole-to-coil ground barrier and method for making same.
A variety of different pole and coil assemblies are known and used for constructing dynamoelectric machine salient poles that are mechanically stable in operation and include ground insulation barriers between an energizing coil and a pole member of magnetic metal. In designing and building such salient pole and coil assemblies several objectives are commonly sought. For example, it is important to assure good structural support of the coil on the magnetic pole member and to provide effective ground insulation between the coil and the magnet metal of the pole assembly. While affording such objectives, a designer must also provide means for effectively dissipating heat that is generated in the coil during its operation. In that respect, it is common practice to attempt to improve thermal conduction between the coil and magnetic pole member thereby to enable the pole member to act as a heat sink for the coil.
In view of such common objectives, currently conventional salient pole and coil assemblies typically have been designed to incorporate dielectric ground barriers and thermal conduction systems between the coils and magnetic pole members in order to provide an optimum combination of the desired characteristics, given the limitations and requirements of the materials available to implement such designs. At the same time there exists a desire to further increase the rated outputs of dynamoelectric machines utilizing such salient pole assemblies. To achieve these desired increases, the field strength of the energizing coils in the pole assemblies must be increased. Such larger field currents impose greater insulating problems on the ground barriers of the systems and also create a need for improved thermal conduction from the coils to adequately dissipate the added heat generated.
A number of different pole and coil assembly structures have been developed in the past to solve the above-mentioned problems while seeking to afford the noted design objectives. For example, pole energizing coils have been wound with insulating tape and then tightly secured around a magnetic pole member by driving wedges between the two components of the assembly to tighten them and improve thermal conduction between them. That sort of relatively expensive and somewhat thermally inefficient system was followed by an improved pole and coil assembly construction in which a coil is wrapped with glass insulating tape then is potted in insulating resin on the pole, to form an electrically insulating shell around the entire assembly. Normally a vacuum impregnation process is needed when practicing such a construction method, in order to avoid the formation of air pockets in the impregnating resin, otherwise, the ground barrier might fail during operation of the assembly. Relatively recently, a system has been developed for mounting a form wound coil on a magnetic pole member then pouring casting compound between those two components to form a rigid structure and to improve thermal conduction between them. Such a method is described in U.S. Pat. No. 3,359,631, which issued on Dec. 26, 1967 and is assigned to the same assignee as is the invention disclosed herein.
It is also known in the prior art to utilize articulated sheets of insulating material mounted between a magnetic pole member and an associated energizing coil. For example, U.S. Pat. No. 830,419 which issued on Sept. 4, 1906 discloses a field coil insulation system that includes a number of formed sheets of impregnated asbestos mounted around an energizing field coil to protect the coil from deterioration due to normal exposure to weather and contaminants such as dirt and water. U.S. Pat. No. 765,189 which issued on July 19, 1904 discloses a transformer coil insulation system that includes a number of plaited corner pieces comprising folded sheets of insulating paper that are arranged to completely cover the corners of the coil and insulate them from an associated magnetic member. U.S. Pat. No. 2,744,204 which issued on May 1, 1956 and is assigned to the assignee of the present invention, discloses an electric coil and associated pole assembly in which spacers are mounted in corner recesses of a pole to complete a stress-relieved rectangular pole configuration so the coil positioned around the pole will form good thermal contact with essentially all of the encompassed pole surface. A coating of plastisol and non-hygroscopic glass mats is mounted around the pole before the energizing coil is positioned on it to hold the glass mats in intimate thermally conductive contact with the pole.
Inherent drawbacks common to such prior art pole and coil assemblies are that they include creased or folded surfaces in the ground barrier insulating materials, particularly where these materials are stretched or folded to form corners in the barrier system. Also, those earlier systems typically required relatively thick layers of insulating mats between the coil and pole in an assembly. Such thick mats undesirably limit the dissipation of coil-generated heat in the pole heat sink.